[coverage] Filter out singleton ranges that alias full ranges
Block coverage is based on a system of ranges that can either have both a start and end position, or only a start position (so-called singleton ranges). When formatting coverage information, singletons are expanded until the end of the immediate full parent range. E.g. in: {0, 10} // Full range. {5, -1} // Singleton range. the singleton range is expanded to {5, 10}. Singletons are produced mostly for continuation counters that track whether we execute past a specific language construct. Unfortunately, continuation counters can turn up in spots that confuse our post-processing. For example: if (true) { ... block1 ... } else { ... block2 ... } If block1 produces a continuation counter, it could end up with the same start position as the else-branch counter. Since we merge identical blocks, the else-branch could incorrectly end up with an execution count of one. We need to avoid merging such cases. A full range should always take precedence over a singleton range; a singleton range should never expand to completely fill a full range. An additional post-processing pass ensures this. Bug: v8:8237 Change-Id: Idb3ec7b2feddc0585313810b9c8be1e9f4ec64bf Reviewed-on: https://chromium-review.googlesource.com/c/1273095Reviewed-by: Georg Neis <neis@chromium.org> Reviewed-by: Yang Guo <yangguo@chromium.org> Commit-Queue: Jakob Gruber <jgruber@chromium.org> Cr-Commit-Position: refs/heads/master@{#56531}
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